Enable commands to be defined in Composer packages

Just followed my own instructions for manually installing a community package:

https://github.com/wp-cli/wp-cli/wiki/Community-Packages

Got a fatal error:

> Call to a member function add_subcommand() on a non-object in /Users/User/.composer/vendor/wp-cli/wp-cli/php/class-wp-cli.php on line 82

The initial idea was to make a home-grown autoloader and use it before loading `vendor/autoload.php`. But it seems there's an easier solution.